Apoptosis is a built-in, signal-induced process by which a cell self-destructs. It is a highly regulated mechanism critical for normal development, tissue homeostasis, and the elimination of virus-infected cells. However, misregulation of apoptosis is associated with human cancer, neurodegenerative diseases, immunodeficiency, and viral pathogenesis. Rational design of effective treatments for apoptosis-associated diseases requires a detailed understanding of the molecular mechanisms by which apoptosis is executed. Importantly, components of the human cell death pathway are conserved with those of less complex organisms, including invertebrates. As such, studies using model insects, including Drosophila melanogaster, have provided fundamental advances in our knowledge of apoptotic regulation. In this application, we outline experiments to establish a comprehensive view of the cellular factors that execute and regulate apoptosis in baculovirus-infected Drosophila cells. Baculoviruses trigger rapid and widespread apoptosis. To prevent premature host cell death, these DNA viruses encode novel and broad-range inhibitors of cell death machinery. The long-term objective of this proposal is to use the experimentally advantageous system provided by baculovirus infection of Drosophila (order Diptera) to define the molecular mechanisms by which a cell executes apoptosis through activation of their caspases and how baculovirus-encoded inhibitor-of-apoptosis IAP and P49 neutralize these death proteases. Our goal is to provide a conceptual framework for the same critical processes in humans and thus contribute to the development of therapeutic strategies for apoptosis-associated diseases. Apoptosis is an important antiviral defense for insects. Thus, it is likely that this host cell death response contributes to the competency with which insects, including dipteran mosquitoes, transmit pathogenic viruses to humans. Our studies are therefore expected to reveal fundamental aspects of insect virus-host interactions that will contribute to the long-term goal of controlling arthropod-borne viral diseases. Here, we use integrated approaches in biochemistry, genetics, and cell biology to define those caspases that are required for virus- induced apoptosis. We use the initiator caspase inhibitor P49 to investigate the mechanism of caspase activation and to design novel substrate-inhibitors that target Drosophila and human initiator caspases. Lastly, we determine the molecular mechanism of viral IAP anti-apoptotic activity by defining interactions between IAPs and pro-death factors. Collectively, these studies are expected to provide new and fundamental information on virus-host interactions during apoptosis of animal cells.